With the development of laser technology, highly accurate optical alignment systems have become a practical reality. As good as these systems are, the various components making up such systems are still subject to vibration, shock, metal creep, and temperature expansion and contractions, all of which leads to inaccuracies in measurements performed by the apparatus. A variety of techniques have been proposed employing complex optical systems to avoid the inaccuracies introduced by the optical components of the system. For example, U.S. Pat. No. 3,522,859 discloses apparatus for measuring the relative positions of a pair of surfaces wherein a specialized optical element is employed to obviate the effects of minor inaccuracies or changes in the position of the support structure for the optical elements of the measuring apparatus. While useful for the purposes disclosed, devices of this type are expensive to manufacture and are not suited for some types of applications where it is desired to determine lateral as well as rotational position or alignment information.
U.S. Pat. No. 3,323,408 discloses an optical alignment system including a light source mounted behind a collimator for projecting a light beam from a reference position toward a reflective target adapted to be mounted on an object the position of which it is desired to measure. The disclosed target may include a first reflective element for imparting lateral displacement information to a return beam and a second reflective element for imparting angular displacement information to a second return beam. The return beams may be intercepted and directed to a photosensitive element by a beam splitter placed within the projected beam formed by the light source and collimator. In devices of this type, the accuracy of the lateral displacement information contained in the return beam formed by the first optical element is a function of the precision of the first optical element. As the distance between the reflective target and the reference position increases, any imperfections in the optical element which result in divergence of the return beam are amplified. Should the distance between the reflective target and the reference position be subject to variation, it becomes extremely difficult to correct errors in measurement without recalibration. Such distance variations are a particular problem when the alignment of machined surfaces in articles of manufacture are optically tested as the articles are moved on a conveyor past an optical testing station.
U.S. Pat. No. 3,790,276 discloses that it is known to sense displacements by projecting an optical beam from a reference location toward a remotely positioned photoelectric sensor. Devices of this type, however, require a photocell array and extremely complex digital electronic processing circuitry in order to produce both angular and lateral displacement information. Somewhat more simplified circuitry is illustrated in U.S. Pat. No. 3,470,377 which discloses an optical position sensing device employing a photosensor array for detecting the pitch and yaw of a remote reflective element arranged to form a return beam projected onto the photosensor array.
In summary, the prior art has failed to disclose a relatively simple electro optical alignment testing apparatus which is both convenient and highly accurate.